xlnation is down at the moment, but it hasn't been closed.
You issue is probably related to something you placed in your city that came with one mod... No need to remove all the files you download, just check in the city what's going wrong!

Oh, it's been a while...
Now, here's some good news: finished some projects and I've got now more time to get back to CitiesXL, in March I bought CXL Platinum - but then because of university, I only managed to install it two days ago!
What I really want to do in the next weeks is updating some of my old mods:
* Unlocker Mod v6 is planned, will support CXL Platinum (I think CXL12 too) and the User Interface Mod. I discovered that most of the features of the old mod are now included in the UIM, so v6 will be just a collection of unlocked buildings (that for some reason are still locked in the game).
* CXL HD v1.5 is under development, you can see two screenshots at the end of this post... Got some problems with tree rendering, but at least it doesn't lage anymore while updating shadows (as it did in CXL11)!
standard screenshot
screenshot with modded settings
See you soon!

It depends...
If the patch is very complex and needs to modify lots of files, probably it needs to modify some of those files updated with CXL12 too... so it will just create problems. However, if it's a simple add-on (like a ploppable building), it will work on CXL12 too.
I suggest reading the comments on that page